Boyle's Law: Air Volume Question

Okay,

I am aware of the basic theory behind the deeper you go, the more air compresses/density it becomes. For example, at 2 ATA (33'), the volume of the compressed air is 1/2 of that at 1 ATA (surface).

What I don't understand is how is it that the air is compressed when it is trapped in a steel tank? Any I looking at this the wrong way. Is it safe to say that as the air leaves my tank and enters my lung, then this is where it becomes compressed?

Thanks for clarifying

The 2nd stage of your regulator "senses" the ambient water pressure around you and delivers the right amount of air to you equal to the ambient pressure.

Your tank is a rigid structure, it's volume never changes. Therefore the air inside it won't compress with depth. The only way for air pressure to change inside a steel tank is if the temperature changes.

I think it's safe to say the air from your tank compresses once it enters your hoses.

Try this, it's a pretty good write up, you can ignore most of the equations and concentrate on the concepts: Kinetic theory - Wikipedia, the free encyclopedia

The pressure of gas in your tank is unaffected by the depth of the water. But the gas coming out of your 1st stage and the breathing gas delivered by your 2nds stage is. Your first stage delivers gas at about 130 psi over ambient pressure. So, at the surface the pressure is about 145 (130 +15) and at 33 ft it is 160 (130 + 30). The gas delivered by your 2nd stage is at ambient pressure so 15 psi at the surface and 30 psi at 33 ft.

At sea level, the ambient pressure is about 15 psi, give or take. For every 33 feet underwater, the ambient pressure increases approximately 15 psi. Since scuba tanks are typically filled to over 2500 psi, they are already significantly more compressed than ambient pressure at any depth obtainable by a diver.

When the air leaves the tank and into your lungs it is decompressed. Starts out at tank pressure and enters your lung at the ambient pressure.

Adam

Think of lungs and your tank as two completely different things. As to your lungs, at sea level it will take 1 unit of air to fill them. At 33' it will take 2 units of air. At 66 feet it will take 3 units of air, and at 99 feet four units of air. For simplicity lets assume it takes 1 cubic foot of air to fill your lungs. (It actually takes alot less). Let's also assume you have a tank containing 80 cubic feet of air. It's all squished into a tank much smaller than that, hence the high pressure in the tank. At the surfance you can take 80 deep breaths off the tank before you run out of air. At 99 feet you can take 20 breaths. The reason is the variable pressure on you lungs, increasing as you go deeper.
Now your tank- it is rigid and not affected by the exterior pressure, so the presuure in the tank drps as air is released from it so you can breathe. So, does that clear it up a little?
DivemasterDennis.

The air gets compressed by the compressor that pumps it into your tank. The pressure in your tank is higher than any pressure you'll ever encounter underwater!

Now, imagine you have taken a drinking glass down to 33 feet with you. It's full of water, but upside down. Now, you take your regulator and carefully purge it, and fill the glass up with air until the air level is right at the bottom of the glass. To do that, you had to push the water out of the way, and you had to push as hard as the water was pushing back, which is the ambient pressure. As already said, this is what the regulator does . . . reduces the very high pressure in the tank, to whatever pressure is surrounding you, whether that's one atmosphere at the surface, or many atmospheres at great depth. Underwater, what keeps the air from expanding to atmospheric pressure is the water, which is used in regulator design to determine the output of both the first and second stages. Regulators are pretty cool that way, and it was the invention of a way to do this that made scuba diving in its current form possible.

From that, you can see that the regulator will drop the pressure from the tank to atmospheric at the surface, but to 2 atmospheres at 33 feet -- so the gas is more dense coming out at 33 feet than at the surface.

Does that help?

Yes, that does help.
Thanks for everyone who responded. I gained a better understanding after reading the post and link:
Thank you again.